1. Field of the Invention
This invention relates genearlly to beam transmission using fiber optics. More specifically, the present invention relates to a transmission path structure which is used to transmit an optical energy beam, particularly a laser beam, over a long distance.
The term "optical energy beam" as used herein means optical beams, particularly laser beams, the energy of which is large enough for use in industries such as for welding or soldering, or in medical treatment such as for cutting away the affected parts of the human body. Thus, this term does not include optical beams for communications.
2. Description of the Prior Art
As is well known, optical fibers comprise a glass body and a sheath enclosing the glass body. The glass body consists of an inner core and an outer clad which is different in reflactive index from the core, so that the light beam is trapped in the core for transmission substantially without leaking from the clad.
Generally, the glass body of the optical fiber used for transmitting an optical energy beam of 10.sup.5 -10.sup.10 W/cm.sup.2 for example have a core diameter of 125-1,250 .mu.m (the clad diameter being about 1.25 times the core diameter), and a length of up to about 200 m. Thus, for transmitting the optical energy beam for a distance beyond 200 m, it is necessary to connect a plurality of fibers by fusing for example. Apparently, such a manner of fiber connection is time-consuming.
When the optical fiber is used for transmitting an optical energy beam (laser beam), particularly the input end of the fiber is easily damaged in a short period because the fiber input end is directly subjected to incidence of the large-energy laser beam (e.g. 10.sup.5 -10.sup.10 W/cm.sup.2). Once damaged, the fiber input end must be cut off to provide a new input end, and the optical components (optical coupler, incident lens, and etc.) associated with the damaged input end must be replaced or positionally readjusted. Obviously, such replacement and re-adjustment is really time-consuming and costly.
In transmitting an optical energy beam (laser beam) into a harmful region such as a radioactively contaminated region, a laser beam source must be located remote from the harmful region for the purposes of safety, and the optical fiber must be correspondingly elongated. In such an application, a portion of the fiber arranged within the harmful region is more likely damaged than the remaining portion. In spite of the fact that the fiber portion arranged within the harmful region is far shorter than the remaining fiber portion, the optical fiber must be replaced as a whole once the shorter fiber portion is damaged, thereby causing a great material loss and taking a lot of time for replacement. Alternatively, the damaged fiber portion may be cut off, and a new fiber portion is connected to the remaining fiber portion. However, such repair also takes a lot of time, necessitating an unacceptably long interruption of the work process (e.g. laser beam welding).
One way to overcome the problems described above is to use a fiber connector for removably connecting optical fibers. However, no practical cable connector has ever been developed which is applicable to optical energy beam transmission fibers.
For instance, an optical communication fiber connector having a pair of ferrules is not usable for optical energy beam transmission because of low light transmittance and/or unacceptable heat generation. Specifically, in such a fiber connector, the connection ends of the fibers are held in direct contact or slightly spaced apart with an intervening matching liquid filled therebetween. When the connection ends of the fibers are held in direct contact with each other, the connection end faces of the fibers are mechanically damaged (scratched at the time of contact, so that the light transmittance between the fibers lowers due to the mechanical damage. Further, a drop in the light transmittance results in corresponding light leakage which in turn is wasted as heat. On the other hand, the matching liquid has a nature of generating heat when transmitting a light beam, and such heat generation becomes particularly remarkable when transmitting an optical energy beam (large-energy laser beam).